World reserves for unconventional oil are estimated to be over 3 times greater than those of conventional oil. As conventional oil sources become scarce or economically non-viable, unconventional (heavy and extra-heavy) oil sources need to be explored in order to supply the world's growing oil demands. However, such unconventional deposits are typically more difficult and expensive to produce.
Heavy oil is a special class of this unconventional oil that has viscosity ranging from about 50-50,000 mPa. Heavy oil reservoirs are often found in high porosity, high permeability, unconsolidated sand deposits. Exemplary oil fields with unconsolidated sands and heavy oils include e.g., the West Sak and Ugnu oil sands in Alaska.
The “West Sak” is the informal name applied to a sequence of oil-bearing very fine to fine grained, unconsolidated sandstones and moderately indurate siltstones and mudstones of Late Cretaceous age (Maastrichtian) that were discovered in 1971 through the drilling and testing of the West Sak No. 1 well. Above the West Sak sands lie the Ugnu sands, also informally named, and containing Maastrichtian Paleocene sands. The Ugnu sands, at depths of 2,000 to 5,000 ft, and the West Sak, from 2,300 to 5,500 ft, both overlay the Kuparuk field (which has been producing since 1981) and under-laying 1,800 ft of Permafrost. The combined thickness of the West Sak and Ugnu formations averages 1,050 ft. These two formations are oil-bearing primarily in the Kuparuk River and Milne Point units.
West Sak is fine to fine-grained sand with interbedded mudstone and claystone deposited as fluvial-deltaic sands. Its porosity averages less than 20%, and the permeability is 10 to 140 millidarcies. It contains intermediate to slightly heavy oil of 50 to 3,000 cP (14′-22.5′ API gravity) at a reservoir temperature of 45°-100° F.
Ugnu is unconsolidated sand. Its average porosity is 25%, and the permeability is very high. It contains viscous heavy oil—that can be classified as bitumen or extra heavy oil of several thousand cP viscosity (7′ to 12′ API gravity) at a reservoir temperature range of slightly lower than 45° to 65° F.
At reservoir conditions, oil may contain dissolved solution gas, thus some oil can be initially recovered using the energy from heavy oil solution gas drive. At the end of primary production, however, a significant fraction of oil still exists for potential secondary recovery.
While there are many secondary recovery techniques, waterflooding is popular because water is usually cheap and readily available. As seen in FIG. 1, water is injected into an injection well to drive oil production towards a nearby production well. Various well arrangements are possible, as shown in FIG. 2, but the basic idea is drive the oil from an area surrounding a production well towards that well.
During waterflooding, water is injected in order to increase the production from oil reservoirs. This is accomplished by “voidage replacement”—injection of water to increase the reservoir pressure to its initial level and maintain it near that pressure. The water displaces oil from the pore spaces, but the efficiency of such displacement depends on many factors (e.g., oil viscosity, displacing fluid chemistry, and rock characteristics).
One of the difficulties with using waterflooding in unconsolidated sands or other “weak” reservoirs is that injection pressures may exceed the strength of rock, resulting in rock breakdown and early water breakthrough. Once this occurs, water typically travels the least restrictive route and no longer functions to drive oil production.